Electronic circuits are generally tested to verify their proper operation; this is often of the utmost importance to ensure a high quality of the production process of the electronic circuits. Tests may be performed on each electronic circuit at different levels of the corresponding production process: for example, a test process called EWS (Electrical Wafer Sort) is performed on the electronic circuits included in a corresponding wafer of semiconductor material, before the individual portions of the wafer on which there are integrated the electronic circuits are diced into individual chips and embedded in respective packages. Such test includes measurements performed on each electronic circuit by automatic test equipment, known as ATE. Such test apparatus is provided with a probe card, which substantially includes a printed circuit board comprising a plurality (typically, hundreds, or even thousands) of probes of conductive material for contacting corresponding conductive pads of the electronic circuits to be tested. During the test, the probe card is positioned in correspondence of a group of electronic circuits of the wafer so that the probes mechanically contact their pads (operation that in jargon is defined as probing); this provides an electrical connection between the test apparatus and the pads of such electronic circuits, which allows the test thereof. The same operations are repeated in succession on each group of electronic circuits of the wafer until all the electronic circuits formed therein have been tested.
In order to ensure that a proper probing procedure has been performed between each probe and the corresponding pad, typically before or after the test, a (manual or automatic) checking operation is carried out which consists of detecting and analyzing a probe mark left by the probe on the pad during the probing operation.
Being able to detect the presence and the arrangement of the probe marks allows first determining the correct alignment and centering between the probes of the probe card on the pads of the electronic circuits under test; in this way, it is possible to reconfigure the parameters of the probing so as to reduce the risk of damage of a passivation layer around the pad caused by a not totally accurate probing. Moreover, such risk is, in modern applications, very considerable, since the number of pads within a same electronic circuit is higher and higher, while their sizes and their relative separation distances are smaller and smaller.
Moreover, the analysis of the probe mark allows determining a correct and uniform pressure exerted by the probes of the probe card on the pads of the electronic circuits, so as to be able to act on the parameters of the probing in order to have a low and uniform contact resistance during the test.
However, in recent applications, the test EWS has drawbacks connected to the impracticality to impossibility of being able to perform such checking operation.
In fact, because of growing demands, especially in automotive applications, of electronic circuits capable of operating in adverse conditions (e.g., high temperatures), the pads (and the corresponding electric connections between the pads and respective terminals of the package) of such electronic circuits are more and more frequently formed by different materials with respect to those traditionally used (such as copper and aluminium). For example, US patent application No. US 2005/0073057, which is incorporated by reference, proposes a multi-layer pad structure having a high hardness property; such property is achieved by using a conductive material with high hardness (e.g., nickel or alloys thereof), possibly covered by a thin protective layer of conductive material that is relatively soft (e.g., palladium or alloys thereof).
The greater hardness of such pads with respect to the materials traditionally used for making the probes results, during the probing, in that the pad is not scratched by the respective probe; in this way, the probe mark is difficult to detect to undetectable, so that it turns out to be very difficult, if not practically impossible, to perform the checking operation of the probing. This involves a difficulty to substantial impossibility to act on parameters of the probing, with consequent increase during the test of the cases of breakage of the passivation layer, and loss of electric yield (due to a high and heterogeneous contact resistance between each probe of the probe card and the corresponding pad), which imply possible false test failures, and correspondingly a reduction in the process yield with consequent increase of the production costs of the electronic circuits.
At the same time, the difficulty/impossibility of leaving the probe mark may be due in part to the mechanical properties of the specific type of probe used for testing. In this regard, various types of probes are commercially available, such as cantilever, MEMS and vertical probes, but their use does not provide satisfactory results in certain situations, making them in practice not always effectively usable.
Typically during the probing it may happen that such probes cause an excessively deep probe mark on the pad such to uncover some layers thereof that, once exposed to the external environment, may be subject to chemical and/or physical changes. This happens for example in the case of nickel, which oxidizes if the palladium protective conductive layer formed thereon is removed, and of copper, which oxidizes if the aluminium protective conductive layer formed thereon is removed. For example, in the case of nickel, the removal of palladium may occur after the use of probes that have a high hardness or after repeated probing actions of the probe on the pad.
In addition, an excessively deep probe mark on the pad typically causes a loss of yield during the assembly operations that connect, for example by welding of wire bonding, the pad of the electronic circuit and the terminals of the corresponding package, as in such situation the welding is not effective; in particular, the effectiveness of the welding is compromised both for geometrical reasons (a highly non-planar surface, such as that obtained in response to an excessively deep probe mark, may not allow a good adhesion), and for reasons of lack of compatibility of the materials involved in the welding (e.g., nickel generally has poor adhesion properties with different materials).